does anyone know why momentum is not conserved in this problem? I thought momentum was always conserved when no net force acted on the system? Is the ground considered the net force acting on the system? Even though it changes direction I was thinking momentum would still be conserved except that the ball would move in the opposite direction. This is the question. Which of the following quantities is (are) conserved when a falling object strikes the ground? I. Momentum of the object II. Kinetic energy of the object III. Total energy Click here to view the periodic table Correct Answer: III only Your answer: You were incorrect I and III only III only Explanation I. Momentum of the object II. Kinetic energy of the object III. Total energy Total energy (III.) is conserved when a falling object strikes the ground. Since the object’s velocity drops to zero after the impact, its momentum certainly changes (eliminating I only and I and III only) and its kinetic energy drops to zero (eliminating II and III only). The total energy of a system (which includes heat) remains constant.